I was previously using Light Flow (mentioned above) on my Droid X and plan on using it on my Nexus too, though I've heard reports it's a bit buggy in ICS still.
I still can't get my notification light to work at all, Light Flow fails to set the bottom LED when I run it in test mode. It does turn on the small indicator (icon) on the top informational row, usually right next to the email icon.
LED worked for awhile but stopped in the last day or so?
Ron W>
If you let Handcent control the LED for text messages and set the flash frequency to default, the LED stays lit in the color you choose. I love it! Haven't found a workaround for the email yet though. I couldn't get lightflow or blink to work correctly.